We can’t yet know whether a new SNP activist base will sweep the decrepit Scottish Labour Party aside in most of the Westminster seats that it holds next May. But there is every likelihood that the nationalists will send more MPs to the Commons afterwards than they do now – perhaps to a hung Parliament.

The SNP is now Scotland’s dominant party. Alex Salmond, Nicola Sturgeon and its leadership have worked to turn it into a force that Scots of all political viewpoints naturally vote for, almost as a form of self-expression. They want it to be to Scotland what Fianna Fail was until recently to Ireland. They show every sign of succeeding.

The referendum “vow” by the three main party leaders to devolve more powers to Scotland, plus Gordon Brown’s referendum intervention, was crafted to halt the SNP bandwagon. It hasn’t done so. But it certainly triggered more demands from Conservative MPs for more powers to be devolved to England.

Many have never been happy with the West Lothian Question posed by Tony Blair and Brown’s devolution settlement (popularly believed to be the imbalance whereby Scottish MPs can vote on England’s affairs, but not vice-versa) nor with the Barnett Formula (a code for higher public spending in Scotland).

Most Tory MPs support some form of EVEL – English votes on English laws. David Cameron rushed out of Downing Street in the wake of the referendum result to proclaim his support for it. He then held a Chequers summit with Conservative backbenchers to calm them down about the “vow” – which many oppose without EVEL coming in to balance it out.

In short, powerful forces are pulling the two biggest parts of the Union away from each other. It is as though Scottish nationalism has hold of the Union’s feet and English nationalism its arms (or vice-versa if you prefer).

The Smith Commission intends to produce proposals which will ease these competing pressures, and put the Union back on a secure footing. However, it deals with Scotland only. Henry Hill previewed it on this site earlier this week.

It must therefore convince the SNP that it leaves the door open to independence before too long, while also persuading Labour that it is actually closing – since Labour is the unionist party that stands most to lose in England from independence for Scotland.

Meanwhile, David Cameron must simultaneously hold out the prospect of EVEL to his party – despite the unlikelihood of this House of Commons voting for it – and claim that the Smith Commission won’t hand yet more powers to Scotland without compensating powers being given to England.

The Commission’s proposals will be published later today, and are trailed in this morning’s papers. The most eye-catching one is that while Scotland will be able to raise its own income tax (up to a point), England won’t have the same power. MPs from Scottish constituencies will still be able to vote on income tax rates for England (and the other parts of the UK).

Some other taxes will apparently remain within Westminster’s control (oil taxes, corporation tax, capital gains tax, national insurance contributions and inheritance tax), while some will not (air passenger duty). Furthermore, altering the personal allowance will be reserved for Westminster, while income tax on earnings will be devolved, but not on savings or dividends. Scotland is also set to gain more control over its welfare spending (the housing elements of universal credit, attendance allowance and carers allowance, the work programme and winter fuel payments). And there is a to-and-fro over abortion law.

Is all this more likely to soothe nationalist sentiment in both England and Scotland, or inflame it – as the various compromises, anomalies, inconsistencies and halfway houses set out by Smith are assailed from both sides? My guess is the latter. England’s localists are already unhappy. Boris Johnson and six Labour mayors want a “comparable package of measures for local government in England”.

The logic of the rise of both nationalisms, and their knock-on effects in Wales and Northern Ireland, leads to a federal UK – the solution supported by the ConservativeHome manifesto. But Labour is fervently opposed to it, since it would reverse the electoral advantage that the party has in England. Nor is the federal solution perfect by any means. England is simply too big a component of the United Kingdom to make federalism straightforward.

I hoped that September’s referendum would settle the Union question for a referendum. What has happened since, and what we know of the Smith Commission’s plan, suggests that this is unlikely.

A last point. What are the consequences for the Union if England votes to leave the EU in a referendum, but Scotland and the other parts of the UK do not?